Methylmercaptan is a well known intermediate for the production of organic compounds, such as sulfur containing amino acids, pesticides and dyes. Industrially, methylmercaptan, also known as methanethiol, is produced mainly for the synthesis of methionine, a widely used feed supplement for poultry.
Methylmercaptan is commercially produced by the heterogeneously catalyzed gas phase reaction of methanol and hydrogen sulfide. For example, EP-B-0832878 and DE-C-19654515 disclose a methanethiol preparation method based on the reaction of hydrogen sulfide (H2S) with methyl alcohol (CH3OH). EP-A-167,354 discloses a synthesis pathway based on the reaction of hydrogen sulfide with carbon monoxide (CO), wherein titanium dioxide (TiO2) was employed as carrier and nickel oxide (NiO) or molybdenum oxide (MoO3) as active component.
Chinese Patent Applications CN 1207957 and CN 1207958 disclose a series of catalysts useful for the methanethiol synthesis from high H2S-containing synthesis gas, wherein the active component (Mo—S—K-based species) comes from the precursor of K2MoS4 or (NH4)2MOS4 plus a potassium salt. In these Chinese patent applications, dimethylformamide [(CH3)2NCOH] and not water is chosen as solvent to dissolve the active component. The described process is hard to handle and expensive. Another disadvantage of the described catalyst and process seems to be the space-time-yield of methanethiol (0.08˜0.19 g·h−1·ml−1cat) which is rather low for a commercial catalyst.
EP-A-104507 describes a continuous process for reacting carbon oxides, sulfur or hydrogen sulfide, and hydrogen at elevated pressure and temperature. The reaction is carried out over a preformed, single-phase, solid catalyst system comprising a porous alumina containing support, on which a mixture of a manganese sulfide and a iron, nickel, zinc, chromium, cobalt, molybdenum or alkali metal sulfide is deposited. The described process is a continuous, vapor-phase reaction in the presence of a specified sulfur-containing or sulfide catalyst system containing manganese to produce methylmercaptan with improved conversions and yields. It is stated that by using the described catalyst system, the methane formation is kept to a minimum, which should result in an improved economic process. Formation of inert by-products, such as methane, should be avoided because these inert materials are difficult to separate from the recycle gases. It would build up in the recycle gas streams and would have to be vented periodically.
Other by-products of the synthesis of methylmercaptan from carbon oxides, sulfur or hydrogen sulfide and hydrogen include carbonyl sulfide, dimethyl sulfide, carbon bisulfide and dimethyl sulfide. Especially carbonyl sulfide formation should be kept to a minimum since carbonyl sulfide is an intermediate in the formation of methyl mercaptan. Low selectivities of carbonyl sulfide result in higher selectivities of methylmercaptan thus improving the overall yield of methylmercaptan and the whole economy of the process. Generally speaking, upon using carbon monoxide as carbon source, CO2 is always formed as a product of the reaction in view of the water released in and carbon monoxide fed to the process. Carbon dioxide formation can be controlled by recycling the unreacted gases and by minimizing the concentration of water in the process.
U.S. Pat. No. 4,665,242 describes a process for the production of methylmercaptan by heating a gas comprising carbon monoxide and/or carbon dioxide, hydrogen sulfide and hydrogen in the presence of a catalyst based on a tungsten sulfide on an activated alumina substrate. In the process, unreacted gas is recycled to the feed gas stream, wherein the water which is formed during the reaction with the catalyst, is removed from the unreacted gas. The desiccation is carried out by passing the gas through a molecular sieve.
Although innumerous attempts have been started to improve the selectivity and yield of methylmercaptan preparation process, there is still a need for further improvements.